How does the email hash matching work?
You can find the technical details where connect.registerUsers is described. This is a more illustrative description. You create a hash of email addresses for users on your site and send it to Facebook. Facebook compares your hash to a hash of all their user’s email addresses and looks for matches. It does this comparison for every email address listed in each user’s Contact Info on Facebook. Initially, only Facebook knows that Jane Smith on your site is the same Jane Smith who’s on Facebook. Your site will know they’re the same person once she accepts the Connect request. This protects her accounts from being linked without her knowledge.
